As for an art of detecting a butt position of two members constituting a weld joint, Patent Document 1 discloses an art that is equipped with a light emitting portion that emits visible light toward a butt portion of members, a photographic device that photographs the butt portion, and butt position means for detecting a dark portion of an image as the butt position, the light emitting portion diagonally irradiating the butt portion on both sides of a butt line in a direction perpendicular to the butt line. Then, in the art of this Patent Document 1, the lighting of light emitters is changed over, and that one of the light emitters which exhibits a high degree of edge clarity at the butt portion is allocated to the irradiation of the butt portion.
Besides, Patent Document 2 discloses an art of diagonally emitting light toward a step portion between works by a slit laser oscillator, receiving this reflected light by a CCD line sensor, and calculating a butt position between the works by a microcomputer on the basis of a position where a light quantity of light received by the CCD line sensor has changed from a high light quantity to a low light quantity.
Furthermore, Patent Document 3 discloses an art of taking an image by an image taking portion that moves in an image taking optical axis direction by a predetermined step amount, and detecting a surface shape while suppressing an influence of multiply reflected light in a light-section method.